Scanning probe microscopes use any of a class of imaging techniques in which a tip that interacts locally with a sample is scanned over the surface of the sample to generate a three-dimensional image representing a property of the surface at different points on the surface of the sample. For example, in atomic force microscopy, the surface interaction force between the probe tip and the sample is measured at each point on the sample. The tip has a very small end and is mounted on one end of a cantilever arm. The other end of the cantilever arm is attached to a cantilever arm mounting structure. The height of this structure relative to the sample can be altered either by moving the structure or by moving the sample depending on the particular microscope design.
As the tip is moved over the surface of the sample, the arm deflects in response to the changes in topology of the surface. The deflection of the arm is measured and used to control an actuator that sets the distance between the cantilevered arm mounting structure and the sample. Images are typically acquired in one of two modes. In the contact or constant force mode, the tip is brought into contact with the sample and the tip moves up and down as the tip is moved over the surface. The deflection of the cantilever arm is a direct measure of force and topographical variations. A feedback controller measures the deflection and adjusts the height of the cantilever arm mounting structure so as to maintain a constant force between the cantilevered probe and the surface, i.e., the arm at a fixed deflection. The height of the cantilever arm's fixed end as a function of the lateral position on the sample is used to construct the final image of the sample's surface.
The applications of the contact mode are limited due to a strong shear force developed whilst the tip is moved over the sample surface while staying in constant contact with the sample surface. These shear forces can damage soft samples. The sample damage can be substantially reduced by operating the microscope in the second mode, referred to as the AC mode.
In the AC, or non-contact mode, the tip and arm are oscillated at a frequency near the resonant frequency of the arm. The height of the tip is controlled such that the tip either avoids contact with the surface or makes only a light intermediate contact over part of the oscillation cycle. In this mode, the tip samples short-range tip/sample forces. The short range forces between the tip and the sample result in changes in the oscillations of the tip. A detector measures a property that is related to the tip position and generates a signal that is likewise related to the position of the tip. This signal will be referred to as the tip position signal in the following discussion. For example, the position of a spot of light on an imaging detector that results from a light beam that is reflected from a mirrored surface on the cantilever arm is used in some scanning probe microscopes to provide the tip position signal.
The controller adjusts the height of the cantilever arm over the sample such that the oscillation amplitude, phase and/or frequency of the tip position signal is kept at a predetermined constant value. Since the tip is not in constant contact with the sample, the shear forces applied to the sample are significantly less than in the mode in which the tip is in constant contact. For soft samples, this AC mode reduces the damage that the tip can inflict on the sample and also provides a more accurate image of the surface in its non-disturbed configuration. This mode is particularly attractive when imaging biological samples.
It should be noted that the image could be constructed using some other parameter beside the height of the cantilever arm as a function of position on the sample when the cantilever arm is positioned to maintain a property of the tip position signal constant. For example, the image can be formed by measuring the amplitude of a harmonic of the tip position signal while the cantilever arm is maintained at a height that maintains the amplitude of the fundamental frequency of the tip position signal constant.
The image is constructed one point at a time and is limited by the rate at which the tip can be moved relative to the sample, as well as by the time required for the servo loop to reposition the tip vertically to maintain the distance between the surface and the tip. The feedback control system that is used to position the cantilevered arm vertically over the sample must extract the needed information from the oscillatory signal provided by the system that tracks the position of the tip as a function of time. The time to extract the information is long compared to the period of the tip position signal. Hence, each point in the image represents an average of a property of the tip position signal over a relatively long period of time.
Accordingly, the time to generate a single image can be several minutes. The scanning time can be reduced if the scan is limited to a small area that contains the structure of interest. In many cases, an optical microscope can, in principle, be used to find the structure of interest and position the probe tip in the region of interest. However, for this strategy to be useful, the structure of interest must be viewable in a light microscope. Many structures have dimensions that are at the limit of the sizes that can be viewed optically, and hence, the microscope must have an objective with a high numerical aperture. Accommodating a microscope with a high numerical aperture and good optical image quality together with an appropriate illumination source within the structure of a scanning probe microscope presents problems.